Circuit arrangements for evaluating capacitive proximity switches usually use capacitive sensor elements, the capacitance of which changes on the basis of actuation. This change in capacitance is evaluated in order to determine the actuation state.
In circuit arrangements which operate according to the so-called switched-capacitor principle, a charging voltage is usually applied to the capacitive sensor element via a switching means, as a result of which a particular electrical charge is transferred to the capacitive sensor element on the basis of its capacitance and the charging voltage. After a charging time, the sensor element is disconnected from the charging voltage using the switching means and is connected to a collecting capacitor or a reference capacitor of a known capacitance via a further switching means, as a result of which charge is transferred from the sensor element to the collecting capacitor. The process of charging and subsequent charge reversal is repeated for a predetermined number of cycles, as a result of which the charge of the collecting capacitor reaches a particular value which is determined, inter alia, by the value of the capacitance of the sensor element. The charge or the resulting voltage of the collecting capacitor is consequently a measure of the capacitance of the sensor element to be measured. Evaluating the voltage of the collecting capacitor makes it possible to infer the actuation state of the proximity switch. After the voltage has been evaluated, the collecting capacitor is usually discharged in a defined manner, and a new measurement cycle can follow.